The use of copper as a conductive interconnect material is favored in semiconductor devices because of the high speed provided by copper. Copper is difficult to pattern and therefore damascene processing technology has been adapted by the industry to form copper interconnect contacts. A typical damascene process may include forming an opening in a dielectric layer, depositing copper over the dielectric layer and within the opening, and then performing a planarization process to remove excess copper over the dielectric layer, leaving the copper inlaid within the opening. Optionally, an etch stop layer is formed on the top surfaces of the copper and the dielectric layer. Thereafter, an upper dielectric layer and a photoresist layer are sequentially formed on the etch stop layer. Lithography and etching techniques are then used to form an opening in the upper dielectric layer and the etch stop layer, exposing an underlying portion of the copper for the purpose of electrical connection.
A challenge associated with the above processes is that the etching process used to etch the via would leave etching residues and by-products. The exposed copper surfaces are prone to oxidation due to these etching residues and by-products, which in turn degrades the quality of the exposed copper surface and increases contact resistance. While a wet cleaning process can be used to remove copper oxides, the wet cleaning process would cause damage to the copper surface.